The present invention relates to a process for manufacturing a cross-linked, expanded elastomeric safety support having a cellular structure comprising closed cells. The support is intended to be mounted on a wheel rim within a tire. The invention also relates to a cross-linkable, expandable blank intended to constitute the safety support in the cross-linked, expanded state. The invention is also directed to the support itself, and to a process for reducing internal heating during travel of this support mounted on a wheel rim following a drop in tire pressure. This safety support can be fitted on tires of two or four wheel vehicles, e.g. motorcycles or cars.
Elastomeric safety supports having closed cells (xe2x80x9ccellular supportsxe2x80x9d) for competition tires which are intended to travel on bumpy courses, such as cross-country rallies, are well known. Such cellular supports permit travel at zero or very low tire pressure by ensuring tire function following perforation of the tire over a distance correlating with the severity of the course conditions.
The cellular supports used for motorcycles are usually toric in shape. Prior to being mounted on a wheel rim, they have a volume slightly greater than that of the interior space of the tire, so that they may occupy all the interior space of the corresponding tires in order to support them at zero pressure.
Such supports are generally prepared by extruding a cross-linkable, expandable rubber composition which has been subjected to thermomechanical working, then vulcanizing and expanding the extruded composition in order to obtain a cross-linked, expanded support resulting from thermal decomposition of a blowing agent which is initially present in the rubber composition.
Under severe conditions of travel at zero tire pressure, the heating of the support results in an increase in the permeability of the walls of the cells allowing the gradual escape of part of the gas contained in the support. As a result, the volume of the support decreases, such that it no longer effectively ensures function of the tire after a certain amount of time has elapsed.
This internal heating can also completely damage the support after a certain amount of time has elapsed. It is then no longer possible to drive the vehicle.
Cellular supports used for automobiles are not supposed to occupy all the interior space of a tire, except when traveling with a flat tire following a drop in pressure, because they are compressed by the inflation air of the tire when traveling at normal pressure. The same disadvantages due to the internal heating of the automobile tire support may occur following a drop in tire pressure.
Attempts have been made in the past to ensure that a cellular support for a motor vehicle can sufficiently dilate inside a tire following a drop in pressure, so as to bear effectively on the tire when traveling with a flat tire. To this end, it has been proposed to provide this support with a fluid that is vaporizable at atmospheric pressure at a temperature of between 29xc2x0 C. and 135xc2x0 C., preferably between 50xc2x0 C. and 90xc2x0 C. Reference may be made to British Patent Specification GB-A-2 013 143 for the description of such a specific support for an automobile.
The rubber composition used for the support of GB ""143 is preferably based on polyethylene foam. These compositions may also be based on a polypropylene or polyurethane foam, on a blend of nitrile rubber and polyvinyl chloride, or alternatively on 1,2-polybutadiene.
The fluid used is supposed to permit expansion of the support when traveling with a flat tire when it vaporizes because of the increase in temperature within the tire. This fluid may be ethylene oxide, methyl chloride, xe2x80x9cFreonxe2x80x9d, mono-alcohols, di-alcohols or water.
It will be noted that this support is not supposed to have minimized internal heating when traveling with a flat tire.
The inventor has surprisingly discovered that the association of water, in an amount of from 3 to 6 phr (parts by weight per 100 parts elastomer), with a diene elastomer having a molar ratio of diene units of less than 15% allows the production of a cross-linked expanded elastomeric support having a cellular structure comprising closed cells that can be mounted on a wheel rim within a tire which, during travel, does not deteriorate and lose volume as a result of excessive internal heating. A diene elastomer having a molar ratio of xe2x80x9cdiene unitsxe2x80x9d of less than 15% is intended to mean a diene elastomer having a molar ratio of xe2x80x9cunits resulting from dienesxe2x80x9d of less than 15%.
Thus, the present invention is directed to a process for obtaining an improved elastomeric safety support for mounting on a wheel rim within a tire. The process comprises kneading by thermomechanical working a rubber composition comprising a diene elastomer having a molar ratio of diene units of less than 15%, water in an amount of 3 to 6 phr, a blowing agent and a vulcanization system, forming the rubber composition into a cross-linkable, expandable support blank of predetermined section, curing the blank in a mold, demolding the blank, and expanding via decomposition of the blowing agent and vulcanizing the cured demolded blank to obtain a cross-linked expanded cellular support. The forming step may be carried out, for example by injection or extrusion.
Advantageously, the rubber composition comprises a reinforcing filler comprising silica in an amount of from 10 to 30 phr and carbon black. The diene elastomer is, preferably, a copolymer of isobutylene and a co-monomer selected from between isoprene and paramethystyrene.